When an article to be processed is to be coated, it is commonly subjected to surface treatment from the viewpoint of ensuring the corrosion resistance and adhesiveness of a coating film. In particular, when a metal (metal material, metal structure) is to be coated, the metal surface is subjected to chemical conversion treatment (surface treatment) for chemically forming a chemical conversion coating film.
An example of the chemical conversion treatment is chromate conversion treatment with a chromate containing composition; however, adverse effects of chromium have been noted. In recent years, a zinc phosphate treatment agent (zinc phosphate treatment) has been widely used as a chromium-free treatment agent (surface treatment agent, chemical conversion treatment agent) (see, for example, Patent Document 1).
However, the zinc phosphate treatment agent is highly reactive due to the high content of metal ions and acids that it contains, hence this treatment agent exerts an unfavorable impact on both the cost and the workability during drainage treatment. In addition, metal surface treatment with the zinc phosphate treatment agent involves the generation and sedimentation of water-insoluble salts. Such precipitates are generally referred to as sludge, and the removal and disposal of sludge generates an additional undesirable cost. Furthermore, the use of phosphate ions is not preferable because they may affect the environment through eutrophication, and the treatment of phosphate ion effluent requires considerable labor. Moreover, metal surface treatment with the zinc phosphate treatment agent requires surface adjustment, which can prolong the total treatment process.
In addition to the zinc phosphate treatment agent and chromate conversion treatment agent, a chemical conversion treatment agent containing a zirconium compound is known (see, for example, Patent Document 2). The chemical conversion treatment agent, including a zirconium compound, contains less metal ions and acids, and hence is not as reactive. This offers a favorable cost advantage and improved workability during drainage treatment. Such a chemical conversion treatment agent is also superior to the above-described zinc phosphate treatment agent in regard to the inhibition of sludge generation.
Patent Document 1: Japanese Unexamined Patent Publication No H10-204649
Patent Document 2: Japanese Unexamined Patent Publication No. H7-310189